Microelectrodes including ion-sensitive microelectrodes will be used in conjunction with isotopic tracer techniques to investigate, in vitro, how the large enterocytes of the amphibian duodenum are able to effect transepithelial secretion of HCO3- or its equivalent) and absorption of Cl-, the way in which these parallel transport processes are linked in the transporting cells and the manner in which the level of cyclic AMP in the cells influences the rate of these parallel transport events. Two hypotheses will be tested. First, that the rate of HCO3- secretion is a function of intracellular HCO3- activity (aiHCO3-) or pH. Secondly, that the level of Cl- accumulation achieved by the enterocyte is a function of intracellular (HCO3-) activity or pH. In order to define the transmembrane driving forces for HCO3- secretion aiHCO3 will be assessed by employing microelectrodes sensitive to HCO3- or pH. In the same way intracellular Cl- activity will also be assessed. Different experimental manipulations will be employed to determine 1) the relationship between intracellular HCO3- activity and the secretory HCO3- flux. The response of villus and intervillus cells to elevation of cyclic AMP will also be determined. 2) the relationship between the level of Cl- accumulation achieved by the active mucosal uptake mechanism and the intracellular HCO3- activity and pH. The mucosal events in electrogenic Cl- absorption will be studied further by measuring the mucosal membrane potential, transepithelial potential and voltage divider ratio while electrogenic Cl- absorption is induced by addition of the anion to the medium. The role of luminal NaC1 cotransport will be examined by measuring labeled fluxes of Na+ and Cl- while electrogenic Cl- absorption is induced. Lastly, to examine the possibility that actively transported sugars stimulate intestinal Na+ absorption by lowering cell K+ the correlation between net Na+ absorption and cell K+ activity will be studied. The objective is to understand the mechanism of active Na+ absorption, active Cl- absorption and the related process of HCO3- secretion as it occurs in the normal intestine and in pathological states such as diarrhea and congenital alkalosis as a prelude to the development of effective therapies against these disorders.